(1) Field of the Invention
This invention relates to rare earth metal-transition metal-boron (R-T-B) permanent magnets and, in particular, to a method for producing such permanent magnets with anisotropy of a bonded type wherein rapidly-quenched R-T-B alloy powder is bonded by binder.
(2) Description of the Prior Art
As an R-T-B permanent magnet alloy, N. C. Koon and B. N. Das disclosed magnetic properties of amorphous and crystallized alloy of (Fe.sub.0.82 B.sub.0.18).sub.0.9 Tb.sub.0.05 La.sub.0.05 in Appl. Phys. Lett. 39(10) (1981), 840 (Reference 1.) They wrote that crystallization of the alloy occurred near the relatively high temperature of 900 K, which also marked the onset of dramatic increase in the intrinsic coercive force. They found out that the alloy in the crystallized state appeared potentially useful as low cobalt permanent magnets.
J. J. Croat proposed amorphous R-Fe-B (Nd and/or Pr is especially used for R) alloy having magnetic properties for permanent magnets as disclosed in JP-A-59064739 (Reference 2, which corresponds to U.S. patent applications Ser. Nos. 414,936 (now U.S. Pat. No. 4,851,058), and 508,266) and JP-A-60009852 (Reference 3, which corresponds to U.S. patent applications Ser. Nos. 508,266 and 544728 (now U.S. Pat. No. 4,802,931). References 2 and 3 disclose that other transition metal elements can be used in place of or in part of Fe. Those magnetic properties were considered to be caused by a microstructure where Nd2Fe14B magnetic crystal grains having a grain size of 20-400 nm were dispersed within an amorphous Fe phase. Reference is further made to R. K. Mishra: J. magnetism and Magnetic Materials 54-57 (1986) 450 (Reference 4).
The rapidly-quenched alloy ribbon is prepared by the continuous splat-quenching method which is disclosed in, for example, a paper entitled "Low-Field Magnetic Properties of Amorphous Alloys" written by Egami, Journal of The American Ceramic Society, Vol. 60, No. 3-4, March-April 1977, p.p. 128-133 (Reference 5.) A similar continuous splat-quenching method is disclosed as a "Melt Spinning" method in References 2 and 3. That is, R-T-B molten alloy is ejected through a small orifice onto an outer peripheral chill surface of a copper disk rotating at a high speed. The molten alloy is rapidly quenched by the disk to form a rapidly-quenched ribbon. Then, a comparatively high cooling rate produces an amorphous alloy but a comparatively low cooling rate crystallises the metal.
According to References 2 and 3, the principal limiting factor for the rate of chill of a ribbon of alloy on the relatively cooler disk surface is its thickness. If the ribbon is too thick, the metal most remote from the chill surface will cool too slowly and crystallise in a magnetically soft state. If the alloy cools very quickly, the ribbon will have a microstructure that is somewhere between almost completely amorphous and very, very finely crystalline. That is, the slower cooling surface of the ribbon farthest from the chill surface is more crystallised but the other quickly cooling surface impinging the chill surface is hardly crystallised, so that crystallite size varies throughout the ribbon thickness.
References 2 and 3 describe that those magnetic materials exhibiting substantially uniform crystallite size across the thickness of the ribbon tend to exhibit better permanent magnetic properties than those showing substantial variation in crystallite size throughout the ribbon thickness.
In order to produce a practical magnet, the rapidly-quenched alloy ribbon is crushed and formed into a bonded magnet. Reference is made to a paper entitled "PROCESSING OF NEODYMIUM-IRON-BORON MELT-SPUN RIBBONS TO FULLY DENSE MAGNETS" presented by R. W. Lee et al at the International Magnetics Conference, held at St. Paul, Minn., on Apr. 29, 1985, and published in IEEE Transactions on Magnetics, Vol. MAG-21, No. 5, September 1985, Page 1958 (Reference 6.)
Generally speaking, the Nd-Fe-B rapidly-quenched alloy can provide only an isotropic magnet because of its crystallographically isotropy. This means that a high performance anisotropic permanent magnet of a bonded type cannot be obtained from the rapidly-quenched alloy. Reference 6 discloses that the bonded magnet has energy product of 9 MGOe or less.
Reference 6 further discloses that magnetic alignment was strongly enhanced by upsetting fully dense hot-pressed samples of the crushed alloy ribbons.
JP-A-60089546 (Reference 7) discloses a rapidly quenched R-Fe-B permanent magnet alloy with a high coercive force. The alloy contains very fine composite structures less than 5 .mu.m predominant of tetragonal crystal compositions and is crushed into powders having particle sizes of -100 Tyler mesh (less than 300 .mu.m) for use in production of a bonded magnet. However, no magnetic properties of the bonded magnets are disclosed therein. Although Reference 7 discloses that C-axis anisotropy was appreciated by application of X-ray diffraction microscopy to a surface of the alloy. However, the crushed powder cannot actually be magnetically aligned.
Sagawa et al proposed an anisotropic R-Fe-B sintered magnet in JP-A-59046008 (Reference 8) which was produced from an ingot of an alloy of R (especially Nd,) Fe, and B by a conventional powder metallurgical processes.
However, the R-Fe-B alloy tends to be oxidized in production of the magnet, because the R-Fe-B alloy ingot comprises the magnetic crystalline phase of the chemical compound R.sub.2 Fe.sub.14 B and the R-rich solid solution phase and because the solid solution phase is very active to oxygen. Accordingly, it is difficult to produce an anti-corrosion anisotropic sintered magnet.
On the other hand, bonded magnets comprises magnetic particles dispersed in and covered with the binder so that the anti-corrosion magnets can be obtained readily. Further, the bonded magnets are simple in a production method in comparison with the sintered magnets and the hot-pressed magnets disclosed in Reference 6.